Evidence accumulated over the past two decades is now sufficient to permit an initial quantitative assessment of the patterns of biotic diversity and extinction that occurred during Proterozoic time. Because of limitations in both the quality and quantity of data currently available, however, generalizations thus derived must be regarded as tentative. Nevertheless, read literally, available palaeontological data appear to indicate that the global ecosystem experienced a gradual but massive collapse between 1 000 Ma and the beginning of the Phanerozoic, a supposition consistent with other lines of geological and geochemical evidence. A possible forcing agent for such a collapse appears to have been a decrease in ambient levels of carbon dioxide and a resultant decrease in average global temperature, photosynthetic efficiency, and primary productivity.